Propagate alignment for _Complex

_Complex load/store didn't have their alignment set properly, which was visible when GCC's torture tests use volatile _Complex.

Update some existing tests to check for alignment, and add a new test which also has over-aligned volatile _Complex (since the imaginary part shouldn't be overaligned, only the real part).
Make x86 fast-isel correctly choose between aligned and unaligned operations for vector stores. Fixes PR16640.